1. Field of the Invention
This invention broadly relates to a crossflow cooling tower having two or more film fill packs located in vertically and horizontally offset relationship to each other, with a water distribution system which divides the water to be cooled into separate streams which are directed to respective upper water inlets of the packs. The invention includes the discovery that there are certain critical parameters for the relative dimensions of the film fill packs so that the effectiveness of the tower can be optimized; specifically, the height of each pack, the length of air travel through the pack and the inclination of the upright air inlet face of each pack should be within certain specific limits. The multi-level fill pack arrangement is especially advantageous as a replacement for splash type fill structure in an existing tower, and has been found to provide a substantial improvement in the duty of the tower.
2. Description of the Prior Art
One imprtant class of industrial water cooling towers has long been those of the mechanical draft crossflow type which have splash type fill structure comprising an arrangement of horizontally extending slats or splash bars. Initially, these crossflow towers had splash bars constructed of redwood because of its favorable cost to useful life ratio under the wet conditions which existed in the tower. Other materials were either more expensive or did not have the longevity attributes of redwood slats. Synthetic resin splash bars began to replace the wooden slats as the cost of redwood rose, and techniques of manufacture and prices of plastics made extruded shapes price competitive with wood.
During the era of wood splash bars for crossflow cooling towers, the first or installed cost of the tower including the splash bar contruction was of primary concern to the engineer during design of such towers, by virtue of the fact that relatively low electrical energy costs for powering the mechanically driven fans did not significantly enter into the overall equation of tower costs when initial as well as operating costs were calculated over the anticipated useful life of the tower.
However, as electrical energy costs have steadily increased over the years, selection and design of industrial water cooling towers has required a comprehensive evaluation of the projected operating costs over the life of the tower in addition to the capital expenses for construction and installation of the tower. An evaluation of future operating costs must also take into account anticipated increases in the price of energy due to inflation. Thus, when a tower owner is faced with a decision as to whether an existing tower should be replaced or reconstructed, the projected operating costs must be assessed along with installation expenses, both for a proposed new tower or for an existing tower having fill structure which must be replaced, in order to reach an advised decision with respect to building a new tower or repairing the old installation.
One alternative to constructing a new tower as a replacement for an existing unit is to change out the fill structure in order to provide a more cost effective means of bringing the hot water to be cooled into sensible and evaporative heat exchange relationship with cooling air directed through the tower by the associated fan structure. However, the fill structure must be more efficient than in the existing tower if the reconstruction is to be justified when operating costs are taken into account. Film fill structure has the potential of providing a more efficient operating package than splash type fill in the same cubic space, but film fill has normally been associated only with package and smaller commercial installations because of the higher initial cost of film fill. Recently, significant improvements have been made in the art of design and fabrication of film fill sheets and as a consequence the thermal effectiveness of the assembled pack has correspondingly increased, thus making installations of film fill packs even more economically competitive with splash type fill towers when the extended time operating expenses for the tower are factored into the calculations.
U.S. Pat. No. 4,548,766 dated Oct. 22, 1985 and assigned to the assignee of the present invention discloses a film fill pack constructed of a series of side-by-side vacuum formed film fill sheets. The fill sheets illustrated and described in U.S. Pat. No. 4,548,766 each have outwardly extending spacers which are complementally received in notches of spacers of the adjacent sheets in order to maintain the sheets in required horizontally spaced relationship.
However, a single film fill sheet pack is not normally a suitable direct substitution for the splash bars in the air-water interchange area of a larger industrial mechanical draft crossflow water cooling tower. One problem in this regard is that if the length of the fill pack is increased to accommodate a larger volume of water to be cooled, the increased cooling effectiveness of the film fill pack is such that a large part the heat transfer between the descending water and the crossflowing currents of air occurs within upper reaches of the fill pack, and a much smaller proportion of heat transfer takes place in the lower regions of the same. This is especially true in large industrial towers because the efficiency of the film fill pack is such that the descending water more closely approaches the wet bulb temperature of the incoming air at a higher elevation in the tower than water descending through a splash type fill structure of a similar tower. Consequently, less thermal interaction occurs in the remaining lower regions of the pack since the thermal driving force is directly proportional to the difference between the water temperature and the wet bulb temperature of the air.
One possible solution might be to increase the width of a film fill pack in an industrial crossflow cooling tower in an effort to improve the overall effectiveness of the tower. In this way, the hot water to be cooled could be distributed over a wider area in order to proportionately reduce the thickness of the water films descending over the sheets. However, such a solution would not overcome the problem of improving the cooling effectiveness of the fill pack within lower regions of the same. Furthermore, the increased horizontal dimension of the film fill pack would have a deleterious effect on static air pressure drop through the fill assembly and in the end, decrease the effectiveness of the film fill. Also, the addition of film fill to extend the width may not be cost effective if the cooling air has expended much of its cooling potential prior to entering the additional fill. Finally, because the film fill is more expensive than a series of splash bars in the same space, it is not cost effective to place film fill throughout the entire area that would normally be occupied by splash bars.